1. Field of the Invention
The present invention relates to an improved process for the preparation of hexene-1,6-dioic acids, and, in particular, hex-3-ene-1,6-dioic acid. Hex-3-ene-1,6-dioic acid is a valuable intermediate that can facilely be hydrogenated into adipic acid.
2. Description of the Prior Art
Adipic acid, one of the starting materials for the production of nylon 66, is produced in vast amounts per annum. By reason of this fact alone, any new route for the synthesis of this diacid and/or derivatives thereof would be of fundamental interest to this art.
U.S. Pat. No. 4,611,082 indicates that the carbonylation of a solution of 1,4-diacetoxybut-2-ene in an aprotic, polar and non-basic solvent selected from among the nitriles, bis(2-methoxy)but-2-ene, bis(2-methoxyethyl) ether and methylene chloride, at 80.degree. to 140.degree. C., in the presence of a transition metal halide is virtually absent, and that, in the presence of an alcohol, the extent of carbonylation increases and is comparable to that observed for the carbonylation of but-2-ene-1,4-diol. With regard to this latter substrate, it is also indicated that satisfactory yields of straight-chain carbonylation compounds are not obtained under the above conditions. Thus, the preferred substrates are substituted in the 1,4- position by alkoxy groups.
Therefore, 1,4-diacetoxybut-2-ene is not considered as a viable promoter substrate for the synthesis of straight-chain dicarbonylated compounds.
Published French Patent Application No. 89/06,018 describes a process for the preparation of hexene-1,6-dioic acids by reacting carbon monoxide and water with at least one butene disubstituted by acyloxy groups in the presence of a palladium-based catalyst and a quaternary onium chloride of a Group VB element of the Periodic Table selected from nitrogen and phosphorus, such element being tetra-coordinated to carbon atoms, with the proviso that the nitrogen may be coordinated to two pentavalent phosphorus atoms.
This reaction can be carried out in N-methyl-pyrrolid-2-one.
The process of the '018 application, which provides appreciable results both in respect of its activity and in respect of its selectivity for a straight-chain dicarbonylated compound, however, presents the disadvantage of requiring the presence of at least one quaternary onium chloride as indicated above. These onium compounds are promoters which are relatively expensive or not readily available and are susceptible to degradation during prolonged use.
Thus, need exists in this art for an alternate process that does not require a chlorinated organic promoter, and particularly one wherein all or part of such organic promoter may be replaced by an inorganic halogenated promoter which is more readily available and relatively more stable over prolonged use.